Take your foot off the throttle in a conventional car and not much happens. You stop accelerating, you start to very gradually slow down, and that’s it. Take your foot off of the gas in a hybrid or an EV, and you’ll notice it. The car slows down much more quickly. Like it’s actually braking. That’s regenerative braking, and this is how it works.
It takes a lot of energy to stop a car. It makes sense, since it takes a lot of energy to get the car going in the first place. A 300 hp vehicle turns gasoline into the equivalent of up to 233 kilowatts of power. Slowing down a vehicle also turns a lot of kinetic energy into something else.
Instead of kinetic (moving) energy, the car turns the energy into mostly heat. That’s why your brakes get hot when you stop. The energy has to go somewhere, it can’t just disappear, so the friction of brake pads on brake rotors causes it to change forms and a great deal of heat dissipates into the air.
But what if instead of wasting the car’s kinetic energy, you could turn it into something else. That’s what regenerative braking does. Turns the kinetic energy into a useful form of energy while slowing your car.
Electric and hybrid cars have electric motors. Electricity flows from the battery to the motor when you’re accelerating or maintaining speed. But if you remove the input power from an electric motor, it works like a generator. Instead of the motor turning the wheels, the wheels are turning the motor. Its how many dual-motor hybrid systems work. One motor is actually used as a generator to turn the engine’s power into electricity, and the other one uses that electricity to turn the wheels.
This electric motor reversal does two things. One, it slows the car. That’s because, number two, it’s making the motor turn the car’s kinetic energy into electricity. That electricity can actually go back to the car’s battery and give it some more range.
The effect in the AC induction motor used in most modern EVs is that the inverter, which changes the DC battery power to the alternating current the motor wants, changes the frequency the motor sees. By lowering the frequency past a certain point, current starts to flow backwards. From the motor to the battery instead of from the battery to the motor.
If you’ve driven a hybrid or an EV, you’ll know that this isn’t a perfect system. You can’t recapture 100 percent of the energy of hard braking. At some point, you’ll need to use the real brakes.
Why can’t regenerative braking brake as hard as the motor can accelerate? A few reasons. Mostly because brakes can turn kinetic energy into heat far faster than a motor can accelerate that car.
That means that the motor would have to turn more energy into electricity than it can turn electricity into kinetic energy. Which isn’t possible. It would also overwhelm the charging system of the battery and even the cables leading to the battery.
So how efficient is it? Audi just sent the e-tron prototype down the hill at Pikes peak to show you. The system can ouptut 300 kW. That’s 402 hp. It can input 299 kW, turning that into electricity to recharge the battery.
Audi says that rate makes it the highest of any production model. So the best rate of regeneration a car can do is about 70 percent. Most cars, especially hybrids, use a much smaller motor. Because they make less power, they’re capable of much less regeneration, and are less efficient at doing it, so give less braking force.
Even that Audi figure, which says that it can recoup enough electricity on the way down to nearly drive back up, has limitations. Once the battery is full, there is no more regenerative braking to be had. From then on, it’s the conventional service brake. At least until the battery runs down again. So crank that AC, radio, and even the heated seats until your regen braking comes back. And if the hill is too steep, some use of the service brakes will still be required.
So that’s how regenerative braking works. And why it can give EV drivers a boost, but no free lunch.